The electrographic recording process, for which the apparatus of this invention is particularly applicable, includes the steps of forming an electrostatic latent image upon a recording medium and subsequently making the latent image visible. The recording medium, usually provided in web form, has a dielectric surface and may be a coated paper, a polyester based transparent film, or other suitable material on which an electrostatic latent image is formed by means of a plurality of writing electrodes or stylii physically positioned on one side thereof to electrically address the dielectric surface as the medium travels therepast through a recording station. On the opposite side of the recording medium there are disposed a series of backup electrodes. When the potential difference between these recording elements is raised to a threshold level, on the order of several hundred volts, an electrostatic charge is deposited on the dielectric surface of the recording medium as the medium passes through the gap between the stylii and the backup electrodes.
Subsequently the latent image is made visible during the development step by applying liquid or dry toner to the recording medium. Normally such electrographic apparatus includes a liquid development system comprising a roller applicator movable through a bath of toner particles suspended in a carrier liquid, or a fountain over which a marking liquid flows. In each case, the recording medium is contacted by a thin film of developer material out of which the toner particles are electrostatically attracted to the regions of electrostatic charge on the medium.
Electrostatic plotters of this type have been commercially successful for many years in a monochrome mode, including a single recording station and a single development station dispensing a single color toner, usually black. More recently, electrostatic color plotters have been available to produce full color plots by the sequential overlaying of a series of separate color images (yellow, cyan, magenta and black) to produce a full spectrum of colors. It is clearly imperative that each color separation image must be in registration with the preceding images so as to prevent color fringes and color errors, and to provide high resolution color prints.
There are two basic approaches to color separation imaging. In one, a series of images are formed each by means of a dedicated recording head and development station. In the other, a single recording head forms each color separation image on the recording medium which is then advanced past one of the development stations. Then the recording medium is returned to the recording head for receiving the next color separation image and is advanced to the next development station. This process of advancing and returning the recording medium through the apparatus minimizes the number of recording heads and obviates the need for their critical alignment with respect to one another. On the other hand, it is very important that great care be taken during shuttling of the recording medium back and forth to insure that it does not mistrack or skew in the apparatus, resulting in color-to-color mismatch.
The web of recording medium, in each case, is dispensed from a supply roller to the image processing stations. In the single pass method, handling of the recording medium is greatly simplified as compared with the multiple pass method, since although a completed plot may be wound onto a take-up spool, it is also possible to feed it out of the machine with no provision for output storage, i.e. feeding onto the floor. This is not the case with the multiple pass method since the recording medium must be under positive control in two directions of movement before a completed plot may be achieved. As illustrated and described in U.S. Pat. No. 4,569,584 advancing and rewinding of the recording medium, in a multicolor electrographic plotter, is accomplished by feeding the medium from a supply roller to a take-up roller and by driving it with an intermediate main drive roller. The supply roller and the take-up roller each are continuously biased in opposite directions by individual drive motors so as to maintain the medium in a taut state of equilibrium which may be overcome by the drive motor applied to the main drive roller. Winding the completed plots onto a take-up spool, as taught in the '584 patent, is satisfactory for many applications but it requires that numerous completed plots be serially wound upon the take-up spool, prior to separating them into the individual plots and delivering them to their respective recipients.
One form of an output spooling station is shown and described in U.S. Pat. No. 4,784,345. In the single pass apparatus described therein, cut sheets of recording media are transported through a document reproduction apparatus and are delivered into a device for automatically rolling the sheets into tubular form. No provision is made for maintaining the medium taut as it is moved into the device.
Often it is desired to obtain an output plot of one's work shortly or immediately after it has been printed on a centralized plotter at a remote location. In such case it would be most convenient if the plotter apparatus had the capability to automatically provide individual plots from the supply web and to deliver them in rolled up, tubular form for ease of handling. It is an object of this invention to provide an automatic spooling apparatus having these features.
It is another object of this invention to provide an automatic shuttling spooling apparatus which will maintain the record medium taut in both directions of movement so as to prevent mistracking thereof.
Yet another object of this invention is to provide an automatic shuttling spooling apparatus which is self threading.